The presently disclosed embodiments relate generally to layers that are useful in imaging apparatus members or components, for use in electrostatographic, including digital, apparatuses. More particularly, the embodiments pertain to an improved electrostatographic imaging member with an overcoat layer comprising a polymeric binder that exhibits low surface energy.
In electrostatographic reproducing apparatuses, including digital, image on image, and contact electrostatic printing apparatuses, a light image of an original to be copied is typically recorded in the form of an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles and pigment particles, or toner. Electrophotographic imaging members may include photosensitive members (photoreceptors) which are commonly utilized in electrophotographic (xerographic) processes, in either a flexible belt or a rigid drum configuration. Other members may include flexible intermediate transfer belts that are seamless or seamed, and usually formed by cutting a rectangular sheet from a web, overlapping opposite ends, and welding the overlapped ends together to form a welded seam. These electrophotographic imaging members comprise a photoconductive layer comprising a single layer or composite layers.
The term “electrostatographic” is generally used interchangeably with the term “electrophotographic.” In addition, the terms “charge blocking layer” and “blocking layer” are generally used interchangeably with the phrase “undercoat layer.”
One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer (CTL). Generally, where the two electrically operative layers are supported on a conductive layer, the photoconductive layer is sandwiched between a contiguous CTL and the supporting conductive layer. Alternatively, the CTL may be sandwiched between the supporting electrode and a photoconductive layer. Photosensitive members having at least two electrically operative layers, as disclosed above, provide excellent electrostatic latent images when charged in the dark with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member such as paper or to an intermediate transfer member which thereafter transfers the image to a member such as paper.
In the case where the charge-generating layer (CGL) is sandwiched between the CTL and the electrically conducting layer, the outer surface of the CTL is charged negatively and the conductive layer is charged positively. The CGL then should be capable of generating electron hole pair when exposed image wise and inject only the holes through the CTL. In the alternate case when the CTL is sandwiched between the CGL and the conductive layer, the outer surface of CGL layer is charged positively while conductive layer is charged negatively and the holes are injected through from the CGL to the CTL. The CTL should be able to transport the holes with as little trapping of charge as possible. In flexible web like photoreceptor the charge conductive layer may be a thin coating of metal on a thin layer of thermoplastic resin.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, however, degradation of image quality was encountered during extended cycling. The complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For example, the numerous layers used in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, an optional blocking layer, an optional adhesive layer, a CGL, a CTL and a conductive ground strip layer adjacent to one edge of the imaging layers, and an optional overcoat layer adjacent to another edge of the imaging layers. Such a photoreceptor may further comprise an anti-curl backing layer on the side of the substrate opposite the side carrying the conductive layer, support layer, blocking layer, adhesive layer, CGL, CTL and other layers.
In a typical machine design, a flexible imaging member belt is mounted over and around a belt support module comprising numbers of belt support rollers, such that the top outermost charge transport layer is exposed to all electrophotographic imaging subsystems interactions. Under a normal machine imaging function condition, the top exposed charge transport layer surface of the flexible imaging member belt is constantly subjected to physical/mechanical/electrical/chemical species actions against the mechanical sliding actions of cleaning blade and cleaning brush, electrical charging devices, corona effluents exposure, developer components, image formation toner particles, hard carrier particles, receiving paper, and the like during dynamic belt cyclic motion. These machine subsystems interaction against the surface of the charge transport layer has been found to consequently cause surface contamination, scratching, abrasion—all of which can lead to rapid charge transport layer surface wear problems.
A common problem occurs when the different layers of a photoreceptor suffer from high surface energy. High surface energy can hinder photoreceptor performance by, for example, reducing the photoreceptor cleanability. High friction with the cleaning blade or a contaminated surface may lead to accelerated wear in the print engine. The higher surface friction obtained in the high energy surfaces can increase the required torque to drive a belt. Sometimes in large high volume machines using long belts and many backer bars the belt can stall. In addition, high surface energy of the top layer may interact adversely with certain toners. For example, it may become difficult to transfer toner from the photoreceptor to paper or an intermediate transfer belt. Thus, maintaining low surface energy is desirable. The wear of the surface generates powder, which can deposit in the machine and cause problems for other components, for example, dirty the optical elements, and spoil the charge uniformity. Excessive charge transport wear is a serious problem because it causes significant change in the charged field potential to adversely impact copy printout quality. Another consequence of charge transport layer wear is the decrease of charge transport layer thickness to alter the equilibrium of the balancing forces between the charge transport layer and the anti-curl backing layer and impact imaging member belt flatness. The reduction of charge transport layer by wear thereby causes the imaging member belt to exhibit downward curling at both edges when the belt functions in a machine. Since edge curling in the belt is an important issue changes the distance between the belt surface and the charging device(s), causing non-uniform surface charging density which does also manifest itself in “smile” print defect in receiving paper copies. Such a print defect is characterized by lower intensity of print-images at the locations over both belt edges. Further, the interaction against developer carrier beads and hard particulate from paper debris which may scratch the surface of the photoreceptor has also been identified to be a major imaging member functional failure, since the scratches may manifest themselves into print defects. Thus lowering the surface energy is always a desired goal.
An imaging member using low surface energy material in the CTL is illustrated in commonly assigned U.S. patent application Ser. No. 11/320,097 entitled “Improved Imaging Member,” to Mishra et al. filed Dec. 27, 2005, and an imaging member using low surface energy material in the anti-curl backing layer is illustrated in commonly assigned U.S. patent application Ser. No. 11/199,842, filed Aug. 9, 2005, to Mishra et al. entitled “Anti-curl Backing Layer for Electrostatographic Imaging Members.” The disclosures of these applications are hereby incorporated by reference in their entirety.
The CTLs of photoreceptors used in current web-based and drum-based machines commonly use MAKROLON or PCZ-300 or PCZ-400 as a polycarbonate binder for the transport molecule. However, these common materials have associated drawbacks. It has further been found that in machines where the CTL is an outermost exposed layer, during cycling of the photoconductive imaging member in electrophotographic imaging systems, the relatively rapid wearing away of the CTL using these materials significantly reduces the functional life of the imaging member. Particularly, in a rigid electrophotographic imaging member drum design utilizing a contact AC Bias Charging Roller (BCR), ozone species attack on the charge transport layer polymer binder is more pronounced because of the close vicinity of the BCR to the charge transport layer of the imaging member drum. For example, rapid wearing away of the CTL can produce debris which scatters and deposits on critical machine components such as lenses, corona charging devices and the like, thereby adversely affecting machine performance. Moreover, in some machines the electrostatic charge builds up due to a high friction coefficient against the cleaning blades which leads to increased torque and scratching problems. These problems can also contribute to future cleaning problems of chemical toners.
Many attempts have been made to overcome the above problems but not without leading to additional problems. For example, in the past, micro particles such as polytetrafluoroethylene (PTFE) and silica have been dispersed in polymeric binders to alleviate the above problems. However, such particles have a stability issue. PTFE forms an unstable dispersion and tends to settle in the mix tanks if not continuously stirred. Non-uniform distribution of PTFE in the CTL can lead to electrical non-uniformity and associate print defects. Also, mixtures of polymers may cause problems of incomparability in solution and phase separation upon drying of the CTL.
Thus, electrostatographic imaging members comprising a supporting substrate, having a conductive surface on one side, coated over with at least one photoconductive layer, may exhibit deficiencies which are undesirable in advanced automatic, cyclic electrostatographic copiers, duplicators, and printers. While the above mentioned electrostatographic imaging members may be suitable for their intended purposes, further improvement on the electrostatographic systems are needed. For example, there continues to be the need for improvements in photoreceptors, particularly for an overcoat layer that protects the charge transport layer and substantially alleviates problems associated with high surface energy.